Impact of elevated nitrogen inputs on seedling growth of five dry tropical tree species as affected by life-history traits

Abstract

We examined whether the responses of dry tropical tree seedlings to elevated nitrogen (N)inputs were associated with functional types, and whether the growth traits of seedlings emerging from seeds of different size within a species were differentially affected by increased N inputs. The study comprised five dry tropical tree species: Albizia procera (Roxb.) Benth, Acacia nilotica (L.) Delile, Phyllanthus emblica L., Terminalia arjuna (Roxb.) Beddome, and Terminalia chebula Retz. Of these, Albizia procera, Acacia nilotica, and P. emblica are pioneer species. The former two are N-fixing legumes. Terminalia arjuna and T. chebula are nonpioneer, nonleguminous species. Albizia procera, P. emblica, and T. arjuna are fast growing, while the remaining two are slow-growing species. Seedlings of these species from large and small seeds were grown at four N input levels (0, 30, 60, and 120 kg N·ha ¹). Height and leaf area were measured periodically. At the end of the experiment (after 4 months) biomass and other growth traits, namely relative growth rate, net assimilation rate, specific leaf area, and root/shoot ratio, were determined. Foliar N and net CO₂ assimilation rates were also determined. The species responded differentially (66% 282% increase in biomass) to elevated N supply, but the response was not associated with between-species seed size variation. However, within species, small-seed seedlings exhibited a greater response. The elevated N input resulted in a greater enhancement in relative growth rate of the slow-growing species. The species response did not follow functional types such as pioneer versus nonpioneer, legumes versus nonlegumes, and deciduous versus evergreen, but rather was individualistic.